RADIO COMMUNICATIONS IN TODAY’S
COMPLEX RF WORLD
Thomas F. Turkington
CoachComm, LLC.
Auburn, AL
INTRODUCTION
This paper explores the use of unlicensed RF spectrum for
wireless production communications. We will look at why
UHF communications is becoming increasingly problematic in
production environments. We will also explain the benefits of
using unlicensed spectrum as well as the associated challenges. In addition we will address Frequency Hopping Spread
Spectrum and other critical digital RF technologies and techniques that may be employed to overcome those challenges.
While available UHF spectrum is decreasing, a growing number of devices are competing for its use. Production demands
are driving the need for more wireless microphones, wireless
IFBs and other UHF wireless devices. To add to the problem,
plans are moving full speed ahead for whitespace device implementation in the remaining UHF TV spectrum. There can
be no debate; the UHF spectrum is becoming more and more
difficult in respect to the use of low power wireless devices.
Why is UHF based communications a problem?
UHF SPECTRUM OVERVIEW
Reliable wireless communication is absolutely essential in
today’s fast-paced broadcast and live production markets,
but licensing requirements, radio spectrum reallocation and
the wide variety of devices competing for clear spectrum
have created an array of problems for system administrators
and end users throughout the world. The traditional UHF
spectrum that has been widely used for wireless intercoms is
becoming increasingly crowded and more difficult to use.
The FCC in the United States, as well as many other agencies
around the world have reallocated UHF spectrum for uses
other than traditional broadcast TV and associated secondary low power auxiliary devices. In the US, approximately
100MHz of formerly available 700MHz spectrum has been
auctioned to communications companies like AT&T and Verizon. These companies are building out infrastructure to use
this spectrum for personal communications and mobile media
applications.
In addition FCC Chairman Julius Genachowski said in October 2009, that there is a “looming spectrum crisis”. Speaking at a telecommunications industry meeting in San Diego,
Genachowski underscored the growing spectrum crunch by
noting that in recent years the FCC has authorized a threefold increase in commercial spectrum while many observers
have anticipated a thirty-fold jump in wireless traffic. In an
interview for C-SPAN's Communicators series on November
20, 2009, the chairman said that there were a lot of ideas
being offered up about what the commission could do to address the need for more spectrum. “We haven't said anything
about which ideas were the best”. He also said the FCC would
be looking at government and commercial spectrum, but that
there were “no easy pickings on the spectrum chart, and hard
choices to make.” Even the most optimistic outlooks call for
large amounts of spectrum to be reallocated from the UHF
spectrum for use in averting this crisis. Several commentators have speculated that much of this spectrum will need to
come from existing UHF broadcast television.
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There are many characteristics of the UHF spectrum that
make it uniquely suited for use in many various applications.
Next we will briefly look at some of the properties of the UHF
spectrum and some common devices that typically utilize it.
UHF spectrum characteristics
The UHF spectrum is technically defined as extending from
300MHz to 3000MHz. For our purposes however, we will
focus on that portion of the UHF spectrum between 450MHz
and 806MHz as this portion of the spectrum has traditionally
been used for wireless microphone and wireless communication equipment. Generally speaking, UHF frequencies have
excellent propagation characteristics. They penetrate walls
fairly well and are less susceptible to multipath fading than
other portions of the RF spectrum. Also, because the length
of UHF waves is not particularly long, between one and two
and a half feet, antenna size is very manageable.
UHF low power auxiliary devices
UHF low power auxiliary devices such as wireless microphones, wireless IFBs and wireless intercom systems have
been in use for decades. The core technology hasn’t changed
significantly in many years. Generally, these devices are now
frequency agile within a portion of the UHF spectrum. Typically the operational frequency of the device can be adjusted
between 18 and 24 MHz in 25kHz steps. They almost always
utilize analog FM technology. These devices incorporate
transmit power levels of between 50mW and 250mW.
Devices that implement this type of technology have operational requirements and limitations that must be understood
for successful deployment. Generally speaking, the minimum
frequency separation between transmitters must be at least
500kHz. In addition, careful attention to intermodulation
interference must be given when selecting frequencies for
collocated use. Because of this, frequency coordination has
become a particularly significant challenge for UHF users
at large shows or events. Frequency coordination software
programs have helped this task to some degree but these
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programs are complicated and expensive and require significant RF knowledge and experience by operators.
Additional challenges for frequency selection when using wireless intercom systems is the required separation
between transmit and receive frequencies. Typically this
separation must be at least 90MHz. This is primarily due to
a phenomenon known as desensing. Desensing occurs when
a transmitter is physically located near a receiver. Desensing
dramatically lowers the receivers ability to detect the desired
transmission thus limiting range and performance. Because
of this, wireless intercoms require two portions of spectrum
to be available and separated by a fixed frequency range.
High power competition
UHF TV transmitters are the dominant, fixed location, high
power sources of interference for low power auxiliary RF
devices. Regulations require that low power devices not
interfere with television reception in any way. Any device
that is found to be interfering with television reception must,
by rule, be turned off or relocated. In addition, as the name
implies, low power auxiliary devices must accept all interference from television transmitters or other licensed users and
have no recourse or remedy other than moving their own
frequency or relocating. Low power devices must normally
stay out of the six MHz television signal if that transmitter is
within 75 miles.
White space devices
A full discussion of white space devices and the implications
they may have on the UHF spectrum is beyond the scope of
this paper. In brief, white spaces refer to frequencies utilized
for TV broadcasts but that are not used locally in a given location. In the United States, white spaces have gained prominence after the FCC ruled that unlicensed devices that can
guarantee that they will not interfere with assigned broadcasts can use the empty white spaces in frequency spectrum.
The term white space is actually a misnomer because many
existing devices such as wireless microphones, wireless IFBs
and wireless intercom systems use this spectrum today.
There has been much discussion of how white space devices
may be implemented and how they would guarantee that
they would not interfere with existing broadcasts or low power wireless equipment. As of the date of the writing of this
paper no decisive conclusions could be made as to if, when or
how white space devices will be implemented and if so what
impact they will have on low power wireless equipment use.
One point is very clear though, broadcasters are concerned
about what white space devices could mean to TV broadcast production in the UHF band. On February 27, 2009, the
National Association of Broadcasters (NAB) and the Association for Maximum Service Television, Inc. (MSTV) asked a
Federal court to shut down the FCC's authorization of white
space wireless devices. The plaintiffs allege that portable,
unlicensed personal devices operating in the same band as TV
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broadcasts have been "proven" to cause interference despite
FCC tests to the contrary. The lawsuit was filed in a United
States Court of Appeals for the District of Columbia Circuit.
The Petition for Review states that the FCC's decision to allow white space personal devices "will have a direct adverse
impact" on MSTV's and NAB's members, and that the Commission's decision is “arbitrary, capricious, and otherwise not
in accordance with law”.
This much we do know. If/when white space devices are
implemented, the potential market size is staggering. The
White Spaces Coalition which is fighting for white space
device implementation boasts companies such as Microsoft,
Google, Dell, HP, Intel, Philips, Earthlink, and Samsung. In addition several consumer groups have joined the White Spaces
Coalition in an effort move white space device implementation through to completion. It seems that given this, the topic
will not go away quietly.
PRODUCTION WIRELESS DEVICES CATEGORIZED
Not all production wireless devices are created equal. Spectrum allocated to wireless devices and required performance
vary based on the function the wireless device performs. We
will now look at various types of production wireless equipment and the requirements for each.
On-air devices
Wireless equipment that will provide audio for on-air use
should be given the highest priority in any wireless plan.
Wireless microphones are the primary elements here. Wireless microphones must provide broadcast quality audio with
extremely low latency. Audio frequency response of at least
40Hz to 12kHz with dynamic range of greater than 90dB are
required. RF hits are not acceptable and as such wireless
microphones must operate with extremely high fade margins
and be given the best portion of the spectrum and be protected from potential RF hits as frequencies are selected.
As it stands at this time, wireless microphone technology that
is acceptable for on-air broadcast use, exists primarily in the
form of analog FM, UHF technology. This means that wireless
microphones are not a good candidate to move out of the
UHF spectrum to help free up available UHF frequencies.
Talent devices
Wireless devices that provide audio to on-air talent are the
next tier of wireless devices. Wireless IFB and In-Ear monitors are the primary elements in this category. These devices
provide program audio and direction directly into the ear of
on-air talent. These devices require high quality audio with
extremely low latency. Audio frequency response of at least
80Hz to 8kHz with a dynamic range of greater than 90dB are
required.
Only very occasional RF hits are acceptable when using these
devices. It is important to operate with good fade margins
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and frequencies with very high expected success rates. At
this point in time wireless IFB and in-ear monitor device technology that is acceptable for broadcast use exists primarily
in the form of analog FM, UHF technology. This means that
wireless microphones are not a good candidate to move out
of the UHF spectrum to help free up available UHF frequencies.
Communication devices
As stated earlier in this paper, wireless communications is
critical for successful broadcast productions. In most cases,
however, these devices can tolerate a higher level of RF hits
than either wireless microphones or wireless IFBs. In addition, a lower level of audio quality is usually acceptable. Audio frequency response of at least 300Hz to 4kHz is required
with a dynamic range of at least 80dB. Maintaining a very
high dynamic range is critical for successful communication
in high noise environments. Occasional RF hits are acceptable as long as they do not compromise communications or
become distracting to the user.
While most wireless intercoms in use today operate in the
UHF band utilizing analog FM technology, there are other
technology options that are readily available and provide high
quality communications. Some of these systems also provide
a higher level of features and functions. These features and
functionality provide users with a more efficient and “wiredlike” experience. Because of this, wireless communications
make the most sense as a candidate to migrate out of the
UHF spectrum when trying to free up more operational frequencies for wireless microphone and wireless IFB use.
MIGRATING WIRELESS COMMUNICATIONS OUT
OF THE UHF BAND
We have now established that of the three major categories
of production wireless devices, wireless communications
is the best choice to migrate to a different frequency band.
Now we will look at specific advantages to making that migration and what is involved.
Alternative frequency bands
If we plan to migrate wireless communications gear out of the
UHF band, we must then decide to what band we will move.
There are several bands that we might consider. In our discussion here, we will limit our scope to four selected bands
that have the most potential for wireless communications use
in broadcast production environments. In frequency order
they are 902 – 928MHz (900MHz), 1880 – 1930MHz (1.9GHz),
2400 – 2500MHz (2.4GHz) and 5725 – 5875MHz (5.8GHz). All
of these bands have advantages and disadvantages that need
to be considered. Let’s take a look at these.
World-wide operation – Availability of use for the spectrum in
locations throughout the world is important for touring and
rental applications. Rules vary from country to country and
not all frequencies may be used in all locations.
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• 900MHz – Limited availability. Acceptable for use in the
USA, some South American countries and a few areas of
Asia. It is not allocated for use in any CE locations.
• 1.9GHz – Acceptable in most areas of the world. Frequencies vary somewhat depending on area. 1880MHz–
1900MHz in Europe, 1900MHz-1920MHz in China,
1910MHz-1930MHz in Latin America and 1920MHz–1930
MHz in the US and Canada.
• 2.4GHz – Acceptable in all areas of the world. The frequency band is basically the same worldwide with some
minor variations in some locations. A few countries limit
operation to a relatively small portion of the band such
as France where the acceptable full power operation in
outdoor applications is 2400MHz – 2454MHz.
• 5.8GHz – Is generally acceptable throughout the world,
but regulations are changing frequently and it is yet to be
determined where worldwide standards will land.
• Actual amount of spectrum available – The amount of
spectrum that is available dictates many factors of an RF
communication system. The number of collocated beltpacks, interference susceptibility, robustness of RF link
and many other factors are all related to the amount of
spectrum that may be used in a given location.
• 900MHz – 26 MHz
• 1.9GHz – 10 to 20 MHz depending on location. In the US
only 10 MHz is available.
• 2.4GHz – 80 to 100 MHz in most locations. Some countries limit operation to less than that such as France
which allows 54 MHz at full power in outdoor applications.
• 5.8GHz – 150 MHz in most locations. Spectrum allocation
for this band is somewhat of a moving target at this time.
Propagation characteristics – How a particular frequency of
RF wave moves through the electromagnetic spectrum and
how it interacts with obstacles and other impediments is important because it will impact things such as range and RF link
reliability. Generally speaking, the higher in frequency you
go, the more difficulty an RF wave has in penetrating obstacles such as walls, human bodies and foliage. 900MHz acts
more like traditional UHF systems while 5.8GHz has a much
higher rate of signal loss due to attenuation and scattering.
This affect makes use of the 5.8GHz spectrum very difficult
for body-worn devices such as wireless intercom beltpacks.
1.9GHz and 2.4GHz both strike a balance between the two
and offer propagation characteristics that are suitable for
body-worn devices, although not as advantageous as UHF.
Regulatory constraints – Regulations for a given spectrum are
important as they impact things like allowable transmitter
power, modulation techniques, interoperability requirements
and other elements that can have a significant impact on
RF system performance and feasibility. Rules governing the
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use of 900MHz, 2.4GHz and 5.8GHz mandate that devices
utilize spreading techniques or other radio technology that
ensures the spectrum can support multiple users simultaneously. While this complicates radio design, it helps to ensure
that the spectrum is available for all users even when there
are many radios in a given location. 1.9GHz has somewhat
similar requirements and also limits the use of that spectrum
to voice communications.
Competition for spectrum use – The number of other devices
that are competing for a given spectrum can have a great
impact on RF link success.
Licensing requirements – The necessity of licensing for use of
radio equipment in a given spectrum is a particularly polarizing issue. It has been said that operating broadcast production wireless equipment in a non-licensed band would greatly
reduce the chance of success due to the increased competition from other non-licensed devices. This is not necessarily
so as even in the current licensed UHF band the presence
of unlicensed radios is rampant. The existing UHF spectrum
regulations require that all users of low power secondary
auxiliary devices obtain a license. The regulations also limited
the ability for non-broadcast entities to obtain a license to
operate radio equipment in this band. These regulations,
however, have done very little to control the use of wireless
microphone and wireless in-ear monitors by bands, churches,
industrial and corporate users. So using a band that requires
licensing does not necessarily ensure that the spectrum will
remain free of non-licensed user interference. All four of the
bands that we are discussing here offer license free operation
in the areas for which the band is approved for use.
Considering all of the factors involved with selecting a suitable area of the spectrum to operate wireless intercom
equipment, the most advantageous band of operation is the
2.4GHz band. This area of the spectrum offers a good blend
of available spectrum, propagation characteristics and regulatory factors.
In review, the 2.4GHz frequency band is a globally accepted
portion of the RF spectrum that is available for unlicensed
use virtually anywhere, world-wide. In the US the 2.4GHz
ISM band is 2400MHz to 2483.5MHz. Elsewhere in the world
the band can vary slightly and can go as high as 2495MHz in
certain countries.
ing a high quality RF link. There is also the added benefit that
all of this comes without any complicated, time consuming
frequency coordination.
UHF wireless intercom systems are spectrally inefficient
Typical UHF analog wireless intercom systems utilize an RF
infrastructure that utilizes multiple frequencies per wireless
intercom beltpack. Depending on type, a UHF wireless intercom system with 8 beltpacks would utilize between 12 and
16 discrete UHF frequencies. Additionally, groups of these
frequencies would need to be separated by large amounts of
spectrum to avoid desensing.
Migrating an eight beltpack wireless intercom system as
mentioned above can open up space for up to 16 wireless
microphones and/or wireless IFBs just by doing a frequency
for frequency replacement. In reality even more wireless
microphone and/or wireless IFBs could be used as these
devices do not have the ridged frequency separation requirements that wireless intercom devices do. This is significant to
virtually any RF plan in a broadcast production environment.
Because of this fact, and because most broadcast production
environments continually require higher numbers of wireless microphones and wireless IFBs, finding a way to relocate
wireless intercom equipment has become a major priority for
many facilities.
THE TECHNOLOGY
Once a decision has been made to move wireless intercom
equipment out of the traditional UHF spectrum and into the
2.4GHz band, it is necessary to select appropriate technology
to ensure acceptable performance given all of the challenges
of operating in an unlicensed environment. There are a multitude of wireless schemes that might be employed to accomplish the task of implementing effective, reliable broadcast
production wireless communications. To explore all of these
technologies is beyond the scope of this paper. As such, we
will concentrate on implementation techniques of commercially available wireless intercom products as of the time of
the writing of this paper. Specifically we will be discussing
one implementation used in a wireless intercom system as it
relates to other in-band devices that compete for use of the
2.4GHz spectrum.
Frequency Hopping Spread Spectrum (FHSS)
Regulations governing the use of this band throughout the
world require that all devices that operate within the 2.4GHz
band use technologies that minimize interference and promote cooperative use of the available spectrum. This allows
multiple devices to operate within the band with minimal
interference or reduction of range and performance. This
stands in stark contrast to older analog UHF radio technologies that have traditionally been used for wireless intercom
systems in the past. This means you can pack a whole lot
more users into a much smaller RF band while still maintainCopyright ©2010 CoachComm, LLC.
The two major types of spread spectrum techniques are
Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). Most devices in the 2.4GHz
band utilize a form of one of these two techniques.
Common 2.4GHz devices that use some form of FHSS are
Bluetooth, machine to machine serial modems, wireless
telephones, some wireless cameras and wireless intercom
systems. Specific FHSS implementation can vary dramatically
from one device to another, but the core FHSS concept is
unchanged.
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First let’s look at what FHSS actually is. FHSS technology
seeks to use as much of the spectrum that is available over
some period of time, but only a relatively small portion of the
available spectrum at any given instant in time. FHSS uses relatively narrow band transmission signals that hop, or change
frequency, periodically. A pseudorandom frequency hopping
pattern is used to determine what frequency will be used
next. This sequence must be known by both the base and the
remote radio. This requires that the remote be synchronized
or “paired” to the base prior to successful RF communication.
In our application for this study, our specific FHSS implementation works like this. Over time the radio system uses
as much of the spectrum as possible, but it only uses about
1.3MHz at one time. This narrow band signal (1.3MHz wide is
actually quite narrow as compared to DSSS signals) changes
frequencies, or hops, 200 times a second or every 5ms. A
relatively simple modulation technique known as Gaussian
Frequency Shift Keying (GFSK) in used to modulate the narrow band carrier. An optimal modulation index is selected to
enhance receiver sensitivity in the presence of Gaussian or
white noise and narrow-band fading.
devices on the RF link share or multiplex the frequency. First
the base radio broadcasts what it has to say to each remote
radio while all of the remote radios listen. Then each remote
radio in turn transmits back to the base radio with its own information. Once all of this occurs the whole system changes
frequencies and it happens all over again. This all takes place
so fast that it seems to the intercom user that his talk and
listen paths are open all of the time. The intercom user can
talk and listen continuously without any interruption in either
direction in true full duplex operation.
Direct Sequence Spread Spectrum (DSSS)
Now let’s look at DSSS. DSSS technology is very different from
the FHSS technology we just discussed. DSSS spreads out the
available RF power over a much wider area of the available
spectrum. DSSS transmissions do this by multiply the data
being transmitted by a pseudorandom “noise” signal that
is much higher in frequency than that of the original signal.
This effectively spreads the energy of the original signal into
a much wider portion of the available spectrum. As a result,
at any given point in the spectrum, the total power at that
particular frequency appears to be much less than if a narrow
band FHSS signal was being used.
The resulting DSSS signal resembles white noise. However,
this noise-like signal can be used to exactly reconstruct the
original data at the receiving end, by multiplying it by the
same pseudorandom sequence used to spread it originally.
For de-spreading to work correctly, the transmit and receive
sequences must be synchronized. This requires the receiver
to synchronize its sequence with the transmitter's sequence
via some sort of timing search process.
Figure 1 – Narrow band FHSS signal
Using a narrow band signal allows the wireless system to
concentrate RF power into a smaller area of the spectrum at
any particular moment in time. This allows the FHSS radio to
maintain a reliable wireless link in the face of RF noise and
interference that could otherwise be a significant problem.
Additionally, changing frequencies very rapidly helps make
the system less susceptible to single point external interference sources, intermodulation and multipath fading (Rayleigh
fading).
FHSS is inherently a one-to-one technology and does not support multiple remote radio access. To enable multiple users
on the same FHSS radio system, a technique such as Time Domain Multiple Access (TDMA) must be used. In older analog
intercom systems each belt as well as the base had their own
individual frequencies. This was easy to engineer, but very
spectrally inefficient and very susceptible to interference and
multipath fading.
By using TDMA, the base radio and all of its associated remote radios operate on the same frequency at any given moment in time. This is called multiple remote access. All of the
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The most common 2.4GHz DSSS devices are Wi-Fi wireless
LAN equipment. Wireless LAN technology comes in various
forms or standards. The most common wireless network
technology in the 2.4GHz band is 802.11b/g. 802.11b devices
utilize traditional DSSS and 802.11g adds Orthogonal Frequency Division Multiplexing (OFDM) for greater speed.
Additional FHSS radio enhancements
In addition to baseline FHSS and TDMA technologies already
discussed, a radio system must utilize other techniques to
ensure that the most robust RF link is established and maintained at all times under varying RF conditions.
Redundant Data Transmit (2xTX) – 2xTX technology allows
a wireless communication system to send all of the voice
communication data twice, once each on consecutive hops.
While this has the effect of halving spectral efficiency, it
increases performance significantly by dramatically reducing
Effective Packet Error Rate (EPER). The loss of one packet
transmission in a harsh RF environment is common. However, because of the pseudorandom frequency relationship of
the consecutive redundant packet transmission, the potential
for loss of any single audio packet (2 consecutive data packet
transmissions) is dramatically reduced. Some form of redunPage 5
dant transmission must be used in any 2.4GHz wireless communication system to maintain clear, intelligible audio even in
harsh RF environments.
Dual antenna diversity – A radio system may incorporate the
use of two antennas on either the base or remote radio side
or both. Typically this technique is employed at the base
radio side due to space and budgetary constraints. Each
base radio antenna acts as both a transmit and a receive
point just as it would in a single antenna design. In a dual
antenna diversity design however, each packet transmission
is sent out from one of the two different antennas. When
combined with a redundant data transmission technique as
discussed above, this approach enables the radio system to
utilize spatial diversity, frequency diversity, time diversity and
polarization diversity for each audio packet being sent. These
diversity techniques greatly improve the chance of each audio
packet being successfully transmitted and received.
Lost Packet Concealment (LPC) – Even in the best designed
and implemented radio system it is inevitable that there
will be some lost audio packets. The use of some form of
advanced LPC technology allows some of the audio packets
to be damaged or missing during transmission while still
preserving the integrity of the audio that the user hears. A
particularly good LPC technique utilizes an A-CELP, Algebraic
– Code Excited Linear Prediction voice compression and lost
packet concealment scheme. This system is a hybrid between
a waveform compression and a true vocoder. The result is
that it requires more than one audio packet loss (at least four
consecutive data packet losses) before any interruption in
audio quality occurs.
Utilizing baseline FHSS and TDMA technology along with all
of the other specific techniques described above allows a
well designed and implemented wireless system to work with
other 2.4GHz devices without the need for frequency coordination. This enables end users to operate wireless intercom
systems even in crowded RF environments without having to
do the time consuming work of an extensive intermodulation
and frequency selection scheme.
and 11 are non-overlapping channels and are the most commonly used Wi-Fi channels for that reason. When channels
1, 6 and 11 are being used together in the same location they
occupy much of the entire 2.4GHz ISM band.
The specific RF characteristics of a Wi-Fi channel vary depending on network data throughput at any given moment.
When the network has very little data moving through it the
RF signal appears to be lower in power and less dispersed.
As data throughput approaches the maximum capability of
the network the RF signal appears to be higher in power and
more dispersed.
The worst case scenario for interference from or to a Wi-Fi
device is when the data throughput of the Wi-Fi network is
very close to maximum. The good news is that by nature of
the way networks function, this doesn’t happen very often.
Typically this type of extremely high data throughput only
happens when very large files are being downloaded or there
is some other high level, constant draw demand on the Wi-Fi
device. Lot’s of users doing normal web surfing activities
on a Wi-Fi network do not typically produce maximum data
throughput conditions.
Utilizing FHSS/TDMA RF design along with the other techniques discussed earlier allows a wireless communication
system to work well even in the face of extensive Wi-Fi
networks. The difference in perceived power on any given
frequency, at any given moment in time allows a narrow band
FHSS signal to penetrate a Wi-Fi signal and be received on the
other end. The 802.11b/g signal appears as background noise
to the FHSS radio receiver. Just as importantly, it works the
other way too. Because of the greater spreading of the RF
signal in Wi-Fi networks, 802.11b/g devices typically will not
see narrow band frequency hoppers as a significant interference source and will maintain over 90% of their throughput
even with multiple FHSS radios present.
MAKING IT ALL WORK
The practical application of utilizing a FHSS wireless intercom
system in the 2.4GHz band in the presence of other 2.4GHz
devices that are competing for space requires some basic RF
knowledge and a good common sense approach.
Interoperation of FHSS and 802.11b/g Wi-Fi systems
There are a total of 14 Wi-Fi channels that may be used
throughout the world. Only 13 of these are used in most
countries. Many countries, including the United States use
only 1 through 11. Each channel is 22MHz wide (versus
around 1.3MHz for narrow band FHSS technology) and most
of the channels overlap each other which prevents their being
used in close proximity to other Wi-Fi devices. Channels 1, 6,
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Figure 2 – 802.11b/g signal at full network throughput
As with all RF systems, the most important factor to ensuring
that both wireless systems work at peak performance is to
get enough physical distance between the two devices. If a
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typically powered Wi-Fi device can be kept at least 50 feet (15
meters) away from a well designed and implemented FHSS
radio system’s components there is an excellent chance that
neither system will see any noticeable interference from the
other. Of course, the more separation the better off both systems will be. Always try to separate RF systems by as much
physical space as is practical.
Operating in a limited portion of the 2.4GHz band
In some cases it may be necessary or simply desirable to limit
the operational band of one or more of the RF devices in the
2.4GHz band. This may be to more effectively share the band,
or it may be to conform to regulations set out by governmental agencies in a specific country or region. It is the user’s
responsibility to select a frequency band that is legal for the
country of operation. Legal 2.4GHz operation varies from
country to country. Many countries allow full use of the 2400
– 2483.5MHz spectrum, but some do not.
Even in locations that allow full band operation you may wish
to choose to limit the operational frequency bands of one or
more devices. This may be the case if you would like to keep
the FHSS wireless system from operating in a given portion
of the spectrum to avoid a specific Wi-Fi device or some
other interference source. Having said that, a well designed
and implemented FHSS/TDMA wireless system as described
above will very likely work well even when collocated with
Wi-Fi or other 2.4GHz devices, so using as much of the band
as your country of operation allows is usually the best idea.
This is because the more spectrum a FHSS radio system has to
utilize, the less impact single point interference sources will
have.
Well designed 2.4GHz band devices should offer multiple
operational bands. This allows the greatest flexibility when
trying to avoid one wireless device with another. Simply limiting a wireless device to the high or low half of the spectrum
probably will not give enough control of the operational band
to be effective when attempting to segregate radio systems.
Antenna selection and placement
One of the most important factors in a successful RF system is
the placement of the antennas relative to the desired coverage area. When using a dual antenna diversity system as
described above, it is critically important that both antennas
be connected and properly located for best RF performance.
Unlike older analog UHF wireless intercom systems that had
dedicated transmit and receive antennas, modern FHSS systems typically have antennas that perform both transmit and
receive functions. Because of this, some technicians believe
it is possible to use one antenna in one location and the other
in a different location. This type of antenna setup is not acceptable because it nullifies the redundant data transmission
scheme employed in the most robust FHSS radio schemes.
This is very important to remember when setting up FHSS
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radio antennas because this means that both antennas are
necessary and equally important. Both antennas must be
used together and properly placed at all times or RF performance will suffer.
Generally speaking, 2.4GHz wireless systems do best when
the base antenna can maintain a direct line of site to the remote antenna. While this is not always possible, an attempt
to set up the system with this in mind will help produce the
best results.
There are some important things to remember when placing antennas for use with FHSS radio systems. Every antenna
has a certain pattern of coverage for which it is useful. When
using a dual antenna diversity system the patterns of both
antennas need to overlap in the desired coverage area to
ensure best RF results. Do not point directional antennas in
two different directions and do not separate omni-directional
antennas too far away from each other
Higher is almost always better when placing antennas.
Maintaining a direct line of sight from the base radio antenna to
the remote is the best possible scenario. The minimum acceptable application of this is to get the base antennas above head
level. In many cases, the best execution is to get the base radio
antennas well above the desired coverage area and point antennas directly down at the coverage area
Choose the location and orientation of antennas carefully.
Omni-directional antennas should be located as close to the
center of the desired coverage area as possible. Position directional antennas on the edge of the desired coverage area and
point them across the area to be covered. Always make sure the
antenna patterns overlap.
SUMMARY
The UHF spectrum is becoming much more crowded and will
continue to do so in the coming years. Production requirements continually call for more wireless devices to complete
the task. Migrating wireless microphone and/or wireless IFB
equipment out of the traditional UHF band is not practical
due to audio requirements and technical limitations at this
time.
Wireless communication equipment can be effectively migrated outside the UHF band using existing technology. The
2.4GHz band is an excellent candidate for wireless communications due to its propagation characteristics, regulatory requirements and amount of actual spectrum that is available.
Spread spectrum technology allows multiple users to share a
given portion of the spectrum effectively without significant
impact on other users. Choosing the right implementation
of spread spectrum technology is important to maximize
operational range and to ensure a continuously robust RF link.
Utilizing FHSS technology for wireless intercom systems helps
to enable collocation with 802.11b/g Wi-Fi networks because
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of the complementary nature of the two technologies. To ensure a robust and reliable RF link is maintained even in harsh
RF environments, other technologies and techniques must be
utilized in conjunction with the base FHSS/TDMA RF scheme.
Eliminating wireless intercom from the UHF spectrum can
provide a vast increase in the number of wireless microphones and/or wireless IFBs that may be operated at a given
location.
ACKNOWLEDGEMENTS
The author would like to gratefully acknowledge the assistance of the following CoachComm team members:
Adam Frank – Product Specialist, Tempest
Debbie Hamby – Director of Marketing
Bob Kean – Senior Hardware Engineer
Louis Loving – Senior Firmware Engineer
Gary Rosen – Global Sales Manager, Tempest
Dennis Vaillancourt – Engineering Manager
© 2010 CoachComm LLC. All rights reserved.
COACHCOMM
Tempest®
205 Technology Parkway
Auburn, AL 36830
Phone: 334-321-1160
Fax: 888-329-2658
www.coachcomm.com
www.tempestwireless.com
Tempest® is a registered trademark of CoachComm, LLC. All other trademarks on the Service are the property of their respective holders. The written and visual
contents of this whitepaper are protected by copyright. You may not reproduce in any form without first obtaining written permission.
Copyright ©2010 CoachComm, LLC.
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